1. Field of the Invention
The present invention relates to a silicon nitride sintered body excellent in mechanical strengths particularly at room temperature and also in productivity and cost, and a process for producing the same.
2. Description of the Prior Art
In recent years, in automobile-related fields, various problems including the preservation of the global environment from CO.sub.2, SO.sub.x, NO.sub.x, etc., have brought about a rapid improvement in the fuel consumption, and an attempt has been made on the use of a ceramic material as a valve material for the purpose of reducing the weight and the friction loss. In particular, a silicon nitride material is the most promising material by virtue of its light weight, high strength, high toughness and high Young's modulus. In order for the silicon nitride material to be put to practical use, it should be not only excellent in the mechanical properties such as strength and toughness but also in conformity with commercial needs in respect of the productivity and cost.
Regarding the silicon nitride material, various research and development have hitherto been made mainly on (1) sintering method and (2) sintering aid, etc., for the purpose of improving the strength. For example, as regards the sintering method, the hot press sintering and hot isostatic pressing under a pressure of 1000 atm or more have been employed. These methods provide a sintered body having excellent strength properties but are not always excellent in the productivity and cost. In order to solve such a problem, a proposal has been made on gas pressure sintering (see, for example, Mitomo, Funtai to Kogyo (Solids Handling Processing Industry), vol. 12, No. 12, p. 27 (1989)). In this method, however, the densification of the final sintered body should be conducted at a high sintering temperature because it accompanies the growth of a B-type crystal grain. This is highly liable to bring about the deterioration of the strength due to the precipitation of coarse crystal grains. For this reason, this method has not been regarded as a technique capable of sufficiently satisfying both the requirements of properties and productivity. Regarding the sintering aid, Japanese Patent Publication No. 21091/1974 proposed A1203 and Y.sub.2 O.sub.3 as sintering aids and these oxides are still now known as typical sintering aids. However, in a sintered body produced through the use of a sintering aid alone, no sufficient densification can be attained without sintering the sintering temperature to 1700 to 1900.degree. C., and in the nitrogen gas pressure sintering near atmospheric pressure, silicon nitride decomposes, so that no stable sintered body can be obtained in some cases. Therefore, the resulting sintered body cannot be regarded as excellent in both the properties and cost.